1. Field of Invention
The present invention relates to a passive infrared sensor that detects the presence of an intruder in a security area by receiving the infrared light that the intruder emits, and in particular to a passive infrared sensor that detects the presence of an intruder intruding through a wall or window of a building.
2. Conventional Art
Conventionally, in this type of passive infrared sensor, the infrared light emitted from a human body is collected by optical components and received by an infrared sensing element, and the angular range (i.e., the detection area) in which the sensor can collect the infrared light when seen in plan view from above is usually divided into plural pairs comprising pairs of pluses and minuses and set. Among passive infrared sensors, there is a “wide sensor”, which is used for the purpose of detecting the presence of an intruder intruding into a wide space such as the interior of a room, and a “narrow sensor”, which is used for the purpose of detecting the presence of an intruder intruding through a window or door facing a narrow hallway. In the case of the wide sensor, numerous (e.g., 5 to 9 pairs) angular ranges of the detection areas are set in consideration of the purpose of use. In the case of the narrow sensor, a small number (e.g., 1 to 2 pairs) of angular ranges of the detection areas are set. Below, the number of one pair of detection areas will be represented by a number.
The detection distance of the narrow sensor is usually set to be longer (1.5 to 2 times larger) than the detection distance of the wide sensor in consideration of the purpose of use. For this reason, with respect to the longest distance (called “rated distance” below) from the position of the sensor at which the sensor can detect a detection target (intruder) to the detection target, the focal length of the lens of the narrow sensor is made longer in comparison to the case of the wide sensor so that the width of the detection target and the widths of the detection areas are the same. There are also cases where the focal length of the lens is left as is, and the rated distance is made longer by increasing the area of the lens (one type of optical component) per one detection area to increase the amount of received light.
However, when the focal length of the lens of the narrow sensor is made longer, the size of the sensor becomes larger, which results in the sensor being conspicuous when it is disposed in a building or the like. Not only is the security effect compromised as a result of the presence of the sensor becoming more easily known to an intruder, but the sense of incongruity of the building increases. On the other hand, when the area of the lens is increased, the width of the detection area becomes wider. Thus, when the movement of an intruder is slow, sometimes the detection thereof becomes difficult. Detection can be facilitated with the design of circuits or the like, but this gives rise to a new problem in that it becomes easier for misdetection resulting from a disturbance or the like to occur.
A passive infrared sensor that can set substantially 180° opposing detection areas with one sensor has been proposed (e.g., see Japanese Patent Application Laid-Open Publication (JP-A) No. 2000-213985; referred to below as “Patent Document 1”). This passive infrared sensor is disposed with an infrared sensing element, a pair of optical components that set mutually substantially 180° opposing detection areas of the infrared sensing element, and a pair of mirrors that cause infrared light from the detection areas to enter the infrared sensing element. According to this passive infrared sensor, the focal length of the optical components can be reduced to ½ that of the conventional format, whereby the size of the passive infrared sensor can be reduced. When the passive infrared sensor is disposed at an intermediate position in a security area, the wiring is also facilitated.
When a passive infrared sensor is disposed outdoors, sometimes misdetection occurs due to a heat source distant from the detection area, direct sunlight, or a small animal entering the detection area, but a passive infrared human body detection apparatus that can reliably prevent such misdetection and detect with high precision only human bodies has been proposed (e.g., see JP-A No. 9-101376; referred to below as “Patent Document 2”).
This passive infrared human body detection apparatus comprises two sensor units, each of which includes a light receiving element that converts incident infrared energy into an electrical signal corresponding to the fluctuation amount of the incident infrared energy and an optical system that collects infrared light and causes the infrared light to be made incident at the light receiving element. The sensor units set predetermined detection areas with the light receiving directions of the optical systems, and change the infrared energy emitted from within the detection areas to electrical signals corresponding to the fluctuation amounts of the infrared energy. The first sensor unit is disposed so that its light receiving direction faces the upper half of a human body to be detected and so that its detection area does not reach the ground. The second sensor unit is disposed so that its detection area is below the detection area of the first sensor unit and faces the ground separated by a predetermined detection distance from the second sensor unit's own disposed position. The passive infrared human body detection apparatus is also disposed with level detection circuits, which output detection signals when the electrical signals outputted from the light receiving elements of both sensor units exceed a predetermined level, and a human body detection circuit, which outputs a human body detection signal when the detection signals have been outputted from both level detection circuits. It is also disclosed that, in this passive infrared human body detection apparatus, the detection distance can be made to conform to the size of the security area by adjusting the vertical orientation of the second sensor unit.
As other prior art, a heat ray detection apparatus has also been proposed which, when plural detection areas with different focal lengths to detection positions are set, lenses with different focal lengths are used for each detection area, whereby the widths of the detection areas are made substantially the same regardless of the detection distance, and misdetection is reduced and reliability is raised (e.g., see Japanese Patent No. 3,324,271; referred to below as “Patent Document 3”).
The above narrow sensors are suited for detecting the presence of an intruder intruding through a wall or window of a building, and there are also passive infrared sensors where the prior arts described in Patent Document 1 and Patent Document 2 are combined and used. The detection areas in this case are formed as shown in FIGS. 6(a) and 6(b), for example. Here, FIGS. 6(a) and 6(b) are schematic descriptive drawings of detection areas in an installation example of a passive infrared sensor 100. FIG. 6(a) is a plan view, and FIG. 6(b) is a front view. It will be noted that, because the detection areas are set to be bilaterally symmetrical around the disposed position of the passive infrared sensor 100, the detection areas at the right side of the passive infrared sensor 100 will be mainly described below.
As shown in FIGS. 6(a) and 6(b), the passive infrared sensor 100 is disposed at a position on a wall surface 50a at a height H (e.g., 0.8 to 1.2 m) from a ground 52 in the center of a narrow security area 51 along a wall 50. The passive infrared sensor 100 has two detection areas, an upper detection area and a lower detection area, each of which comprises one pair (+ or − will be added to reference numerals below) of divided areas in the horizontal direction. Upper detection areas 101+ and 101− are set facing the substantially horizontal direction from the passive infrared sensor 100. Lower detection areas 102+ and 102− are set facing slightly above the ground 52 at a position separated by a rated distance L1 from the passive infrared sensor 100. When these detection areas are seen in plan view, the detection areas 101+ and 102+ overlap, and the detection areas 101− and 102− similarly overlap. When the length of the security area 51 is short, e.g., when the distance from the passive infrared sensor 100 to the end of the security area 51 is a distance L2, the presence of an intruder in the security area 51 actually serving as the target can be more reliably detected by changing the lower detection areas 102+ and 102− so that they are set to face the vicinity of the ground 52 at a position of the distance L2 from the passive infrared sensor 100.
Even if the passive infrared sensor 100 is suitable when the width (where the widths of the pair of detection areas are combined) of the detection area at the end of the security area 51 is the detection width of the intruder, sometimes the width of the detection area becoming narrow becomes a problem at close range. For example, if L1=6 m and L2=2 m, the detection width is suitable if the width W1 of the detection area is 0.6 m at the end of the security area. However, the width W2 of the detection is 0.2 m at the distance L2, which is too small for the detection width, so that the detection performance drops. In order to reliably detect the presence of an intruder in such a case, the detection sensitivity of the passive infrared sensor 100 may be raised, but when this is done, there is the possibility to create the problem of a false alarm resulting from a disturbance or the like. It is also conceivable to add an infrared sensing element or an optical system and set a new detection area dedicated to close range, but this results in increases in the size and cost of the passive infrared sensor 100.
In the prior art of Patent Document 3, plural lenses with different focal lengths become necessary, and detection areas with appropriate widths can be set in accordance with the distance to an intruder, but the configuration becomes complicated, which results in increases in the size and cost of the sensor.